(1) Field of the Invention
The invention relates to sorting apparatus for sorting objects, input to the apparatus as a stream of objects, according to one or more object characteristics such as size, shape, object-material and colour. The invention relates especially, although not exclusively, to the sorting of objects in a waste stream (for example a stream of household or industrial waste) according to material-type of the objects.
(2) Description of the Art
The sorting of objects within an input stream according to a particular characteristic or set of characteristics, for example, material-type and colour, is typically a first step in a recycling process or indeed could be applied to most other processes which involve sorting of objects on a moving conveyor, such as in the food, assembly and other industries. Such sorting often involves inputting a stream of objects to a conveying means which conveys objects in the input stream past a sensing means. The sensing means identifies objects belonging to particular object-classes (each object-class being defined by one or more object-characteristics) and determines the positions of identified objects across the input stream. Data relating to object-classes of objects and their positions across the input stream is output by the sensing means and subsequently used by extraction means which physically removes identified objects from the input stream to locations (e.g. storage containers) each of which corresponds to a particular object-class. An example of this type of sorting is disclosed in U.S. Pat. No. 6,144,004. Typically, this removal process is based on an estimation of an object position on the conveyor following the sensing of a particular property at a known time on a conveyor travelling at a set velocity.
The conveying means in sorting apparatus is typically a conveyor belt. One known method of removing objects which have been identified and located from a moving conveyor belt involves use of an air-separation unit comprising a single linear row of upwardly-directed nozzles positioned at the end of the conveyor belt, and arranged substantially at ninety degrees to the direction of motion of the surface of the conveyor belt, and through which air may be blasted to produce a plurality of air jets. Each air jet is provided through an individual nozzle, and linear groups of one or more individual nozzles may be activated to provide groups of air-jets. If an object belonging to a certain object-class has been identified, knowledge of the object's position and speed (assumed to be equal to the conveyor belt speed) may be used to activate a linear group of nozzles lying in the path of the object at an appropriate moment to eject the object from the input stream, and into a receptacle. Such an arrangement may be extended to provide extraction, and hence sorting, of objects belonging to two (or more) object-classes at a single point by providing a further, downwardly-directed linear array of air jets, and further receptacles for receiving objects belonging to other object-classes. As an alternative to sorting objects belonging to a plurality of object-classes at a single position, a series of separate conveyor belts, each having a linear nozzle-array positioned at its end, may be employed to give a serial array of binary sorting positions spaced apart by conveyor belts: at each sorting position objects belonging to one object-class are ejected, with the remainder of the input stream passing onto the next conveyor belt. Such arrangements suffer from the disadvantage of requiring sensing of the objects to be carried out at each stage. Examples of these types of sorting apparatus are described in “A Review of Automated Technology to Sort Plastics an Other Containers”, a report produced for the Canadian Plastics Industry Association and ‘Corporations Supporting Recycling’ (CSR, based in Ontario, Canada), and in published international application PCT/GB03/00141.
A problem associated with these types of sorting apparatus is that an air-separation unit comprising a single row of nozzles arranged substantially at ninety degrees to the direction of movement of the conveyor belt provides very unreliable ejection of objects. Also, those identified objects which are successfully ejected are ejected with widely-varying trajectories. This may lead to ejected objects colliding with each other in flight, missing collection receptacles and even falling back onto the conveyor belt, all of which reduces the efficiency of the sorting process. Furthermore, the problem of inconsistent trajectories makes sorting of objects belonging to several object-classes at a single sorting position very difficult because inconsistency of trajectory can easily result in an object falling into a receptacle that does not correspond to the object-class of that object. This means that reliable sorting into a number of object-classes generally has to be carried out using the second type of apparatus described above, viz, a serial array of conveyor belts each having a linear air separation unit at its end, i.e. apparatus having a series of binary sorting positions. This results in long, complex and expensive sorting apparatus.